A typical constraint on the measurement capability of a thermocouple (TC) is the oxidizing point of the base metals. In most cases, when a bimetallic structure is exposed to temperatures above the manufacturer's recommendations, the materials begin to breakdown. This inevitably degrades the welding joint and eventually destroys the circuit.
Aerospace researchers are utilizing advanced modeling and simulation techniques to accurately measure the effects of common thermo-mechanical loads present in common flight profiles. To increase model fidelity, precise dimensions must be applied to the model for the results to match the physical specimen. Installation techniques provided by TC vendors detail procedures for their products to adhere to the surface, but the amount of adhesive used can alter the dynamic response of the material being characterized. Additionally, variability of the TC placement within the encapsulating material, and variability of the TC with respect to the test specimen, can impact data integrity. Excessive adhesive shields the substrate from exposure to ambient heat sources, thus skewing results. Furthermore, irregularities in the geometry or thickness of the adhesive patch used to encapsulate the TC will degrade data accuracy.
There are several methods known to the art for attachment of high temperature TCs. While ceramic adhesives may generally adhere to composite structures, the attachment techniques often varies from project to project. Ceramic adhesive instructions provide proper mixing ratios and curing temperatures and times but, the amount of adhesive to use is left to the user's discretion. Because of this, an unknown amount is often applied to the attachment site, and this amount varies from application to application (and may vary as a result of different technicians' preference).
This technique presents several problems. First, a larger amount of adhesive results in an increased surface area coverage of the specimen. This is known to induce varying flux rates, thus altering the resultant thermal test data. In addition, without a standardized size and shape, correlating the data to the adhesive geometry becomes difficult. An accurate finite element analysis (FEA) is predicated on known material properties and the dimensions through which they interact. An undefined shape can only be estimated with relatively poor fidelity.
An improvement over the above noted method is to protect the TC bead by placing the bead inside a rectangular cap of known dimensions made from Carbon-Carbon. These caps may be attached to composite structures using a graphite-based adhesive applied to the edges and a generous amount injected into the underside cavity of the cap.
While these structures provided more accurate dimensions for FEA, their size and material properties limited their use. A device that produced a smaller footprint and could be applied to a wide variety of structures (both composite and metallic) was required in order to create a laboratory-standard method for TC attachment. This is especially true when the mass of the cap is a non-trivial ratio of the specimen's mass.
Therefore, there exists a need in the art for a method to encapsulate a miniaturized TC with a consistently dimensioned and positioned adhesive barrier at a desired attachment site, thus increasing analysis capabilities and extending the TC life at extreme temperatures.